Motion picture projectors, with or without accompanying sound, have been used for many years; such projectors have used two exposed hubs, often with both hubs actually located external to projector body. In some cases, non-sealed cassette has been disposed outside the projected body and an exposed take-up hub located within the non-sealed projector body. In either case, the exposed film must be threaded through the optical projecting system and the leading end affixed to the take-up hub.
In certain applications, it is desirable to provide a miniaturized, rugged projection system which not only is simple to operate, but also capable of operation, storage and transport in almost any climatic condition to be encountered throughout the world. These objectives can be met by using a completely sealed film cassette which can be inserted rapidly into, or removed readily from, the projector and by mounting the cassette within the projector in such a manner that the film is moved within the sealed cassette by drive means within the projector which is external to the film cassette.
The projector has a relatively narrow hinged and latchable cassette cover which can be opened to provide a narrow access opening in which to insert the sealed film cassette. The cassette can be readily, simply, and accurately positioned with the projector by placing its alignment key into an alignment key slot in the projector. The keyed cassette cover then is pressed against the partially inserted cassette until the cover latch engages a latch on the projector body. There are driven connectors in the projector which interface mechanically with the cassette film control driving means for the film rewind and take-up hubs, and with the intermittent and continuous film drive sprocket wheels. This interface is accomplished by rotatable connectors which extend through seals in the periphery of the film cassette and are biased in engagement with the driven connectors in the projector by the latching of the hinged cover of the projector to the projector body, in conjunction with the pressure exerted on the aforesaid engaging connectors by leaf springs in the cover along one edge of the aforesaid access opening.
In prior film projectors, the optical projecting beam(s) which is directed through the film normally passes between the two film hubs with the axis of the beam(s) lying substantially in a plane passing through the two hubs. With such an arrangement, it can be shown that the overall space required to receive the film reels on two such hubs must be four times the radius R of a fully wound reel of film. In addition, a considerable space of width must be provided between the points of maximum excursion of film on each film reel to permit uninterrupted passage, at all times, of the optical projection and optical sound extraction beams through the reels of film. In summary, a minimum width of 4R+ d would be required. It is important that the space occupied by the reels of film and the optical projection equipment be kept to a minimum. In accordance with the invention, the film is twisted through ninety degrees in the region of the two optical projection beams in order to obviate the need for projecting the optical beams between the reels of film. Also, in accordance with the invention, the film after being removed from the rewind hub onto the take-up hub, subsequently can be driven in the reverse direction back onto the rewind hub. In this manner, the maximum spacing between the two hubs is substantially equal to the radius R of a completely full reel of film, as contrasted with a spacing= 2R+d for the conventional projector. Since the maximum excursion of periphery of the full reels of film from the empty hub is equal to the radius of a full reel of film, it is obvious that the minimum cassette width needed to just accommodate the film in the projection system of the invention is approximately 3R, as contrasted with 4R+ d for the usual film projection systems of the prior art. If one were, for the sake of argument only, to attempt to operate the projector system of the invention without the aforesaid ninety degree twist in the film, it would be essential to introduce a group of mirrors within the sealed cassette; this would complicate construction of the film cassette and alignment of the cassette within the projector and would require undue enlargement of the cassette in order to accommodate the mirrors.
Certain other advantages of the projection system design of the invention can be summarized as follows:
The film remains completely enclosed in the sealed film cassette during transport, presentation, and storage, and is never directly exposed to harmful environmental conditions such as dirt, dust, moisture and the like. The projector mechanism does not physically contact the film at any time. Because of this fact, and because the operator cannot accidentally touch, jar, jam, scratch, or otherwise mutilate the film, film image quality and film sound quality can be maintained reliably at a high level for a long time. The usable life or the film also can be increased because special lubricants can be sealed along with the film in the cassette under controlled conditions in the manufacturer's plant or in the laboratory.
The projector is extremely simple to operate since the operator does not come into direct contact with the film. Moreover, no delicate film threading or film-gate or aperture cleaning operations are required.
The projector design permits the film cassette to be changed rapidly-- in less than 10 seconds. The projection cassette design inherently permits fast forward and reverse winding of the film, in addition to offering the capability of operating at all standard film projection speeds.
Since each film cassette inherently contains its own film take-up and rewind hubs, immediate rewinding of the film need not be done in the case of a multi-reel presentation, but can be done at the conclusion thereof; this is an important factor where multi-reel presentations are required and only a single projection device is available.
An intermittent film advance movement is needed to move the motion picture film through the film projector gate of the projector. Each film image frame must be moved rapidly into the projection gate position and then held steadily for projection in that position for a discrete time interval. The cycle then is repeated by rapidly advancing the next film image into position. Current intermittent film advance mechanisms used in projection equipment normally use either a mechanical claw or a "Maltese cross" type of movement. All of these movements are essentially mechanical cam and pin systems which require a minimum of two moving parts with considerable dynamic mechanical contact and interaction therebetween. High speed intermittent movement, as now used on projectors, would cause damage to the film by placing reverse stress in the film perforations and would rip the film stock. Moreover, mechanical wear of such conventional intermittent movements at high speed would be excessive. In accordance with the invention, the desired film advance intermittent movement is obtained by simple electronic control of a stepping motor. Each incremental stepwise rotation is accomplished by applying an electronic pulse to said motor. In the case of sound film, the film must be driven in continuous fashion at relatively constant speed past the sound extraction beam location of the projector. The use of low voltage stepper motors provide incremental rotary response to thousands of pulses per second. The stepping motor can be advanced by a symmetrical train of such electronic pulses to provide relatively high speed continuous film movement. A full range of pulse rates can be obtained readily by miniaturized electronic circuitry to provide any number of desired continuous operating speeds. The stepping motor allows for both intermittent and continuous motion in the forward or reverse direction and allows for safe, high speed movement of the film through the projector. In this way, rapid access to any portion of a lengthy film reel can be had. The overall reliability of the projector is increased by use of the stepping motor, since no dynamic interaction of mechanical parts is required, as in the case of conventional pin and cam type projector movements. A reduction of parts, as well as spare parts inventory for projectors is possible, since one type of stepping motor can be used to provide the film advance function and the rewind function. Moreover, the electrically controlled stepping motor can operate from low voltage battery supplies.